1. Field of the Invention
The subject invention relates to a speed control device for controlling the rotational speed of DC electrical motors.
2. Description of the Related Art
For a given applied voltage, and presuming no mechanical load, electrical DC motors generally rotate at a relatively constant speed. However, many practical applications require control of the rotational speed of the work shaft which is rotated by the motor. Therefore, various methods have been developed to vary the rotational speed of a work shaft rotated by the motor. Examples of such methods are reduction gears or belts. However, such mechanical systems generally; require much space, generate noise, and are susceptible to mechanical failures. Accordingly, various ways to control the rotational speed of the motor shaft itself have also been developed. For example, one known method for controlling the speed of an electrical motor is pulse width modulation (PWM). While PWM can provide accurate control of the speed of the motor, it requires relatively elaborate circuitry, generally including one or two timers. Therefore, it is not particularly suitable for low cost applications.
In attempting to control the speed of a DC motor one encounters two problems. First, the speed of the motor tends to change during its operation depending on the mechanical load imposed on the work shaft. For example, the speed at which the shaft rotates on a portable drill changes depending on the pressure applied to the drill by the user of the drill. A second problem is that the speed of the motor also changes depending on the voltage level of the battery used to energize the motor.
These two problems are particularly difficult to solve when one attempts to design a low cost apparatus using the motor. However, in certain low cost applications, accurate control of the motor is very beneficial. One example of such a low cost application is a portable metal inert gas (MIG) arc welder. With respect to MIG welders there is a need to control the speed with which the thin wire electrode is delivered. Accurate control of the wire delivery speed enables one to make accurate welds for work pieces of various thickness. Moreover, once the appropriate speed has been established, the user typically expects the motor to maintain a constant speed for the duration of the welding process, irrespective of the battery voltage level. Furthermore, once the appropriate speed has been set, the user expects the motor to rotate at the set speed at all future welding sessions, regardless of whether a different battery is used at the future welding sessions. For example, a user may have a shop power source set-up which is fully charged and a field power source set-up which has been partially discharged. It would be very beneficial if, for a given speed set-up, the wire delivery rate would be the same for the shop and the field power source set-ups.
In addition to the above noted problems, another problem which is common to MIG welders is that the arc welding process causes sudden supply voltage changes and interference within the welder. For precise electrode delivery speed control, the speed control circuit for a MIG welder must be able to overcome these sudden changes and interference.
Therefore, some welding systems use separate and independent power systems for the creation of the arc and for the electrode delivery. Such a system is disclosed in, for example, U.S. Pat. Nos. 4,801,780 and 4,703,156, both to Hayes. These patents disclose a configuration wherein the welder power supply generates the arc, while a conventional hand-held drill drives a gear mechanism which delivers the electrode.
Another speed control MIG welder is disclosed in U.S. Pat. No. 5,086,208 to Habermann. This patent discloses a variable resistor in conjunction with a diode bridge for controlling the speed of the motor. However, the variable resistor system suffers from changes in the speed of the motor as the battery voltage drops during a welding session. The variable resistor system further suffers from speed inconsistencies when different power source set-ups are used.